In general, a hybrid electric vehicle (HEV) is a vehicle which uses two types of power sources, and the two types of power sources are mainly an engine and an electric motor. Such a hybrid electric vehicle has excellent fuel efficiency and power performance and is advantageous in reduction in exhaust gas, as compared to a vehicle provided with an internal combustion engine alone, and thus has been vigorously developed.
A hybrid electric vehicle may be operated in two driving modes according to types of powertrains which are driven. One is an electric vehicle (EV) mode in which the hybrid electric vehicle is driven using only the electric motor, and the other is a hybrid electric vehicle (HEV) mode in which the hybrid electric vehicle is driven by operating both the electric motor and the engine. In a hybrid electric vehicle employing a parallel type hybrid system in which an electric motor and an engine clutch (EC) are installed between an engine and a transmission, the engine clutch is in an open state in the EV mode and is in a closed state in the HEV mode. The closed state may be divided into a slip state in which slip between clutch plates is allowed and thus RPMs (revolutions per minute) of an input terminal and an output terminal may be different, and a lockup state in which RPMs of the input terminal and the output terminal are the same through direct connection of the clutch plates.
However, fixture of the engine clutch to an arbitrary state (hereinafter, referred to as “stuck” for convenience of description) due to mechanical failure or an error in control may occur. Particularly, stuck becomes worse if, when the state of a dry engine clutch is changed for improvement of fuel efficiency, a self-locking structure for maintaining the corresponding state without separate control is employed.
If such stuck or failure in which the state of the engine clutch cannot be determined occurs, the general hybrid electric vehicle is controlled so as to be driven using only the electric motor in all cases. However, such control forces the hybrid electric vehicle to be driven using only the electric motor regardless of the state of the engine clutch or a driving load and, thus, in some cases, may cause rapid battery consumption and difficulty in securing a sufficient moving distance of the hybrid electric vehicle to a safe zone in the event of failure and increasing the risk of accident.